The present invention relates to a method and an apparatus for separating blood and gas and particularly to moving blood through a non-rotating centrifuge for separating blood and gas.
It is useful medical procedure to draw blood from the site of a patient's wound and then introduce the drawn blood back into the patient or into another patient. The wound may be due to surgery or accident. As blood is withdrawn, it is common for air to inadvertently be introduced into the blood at the site of the withdrawal. However, blood that contains air must under no circumstances be admninistered to a patient. Therefore, air must be separated from the blood before it is introduced or re-introduced into a patient. Furthermore, blood is damaged by inclusions of air the longer the drawn-in air remains in the blood. Still further, blood is damaged the more that air is mixed with blood within the blood-removal path. Therefore, it is desirable to separate the air from the blood as close as possible to the site of the removal or withdrawal from the patient.
The quality of blood can be impaired among other things by the amount and length of presence of air or other gases in the blood; by suction or pressure forces on the blood; by frictional forces of the blood in flow paths, and by deflections of the flow of blood and turbulence in the blood.
There are several situations in which gas must be separated from blood. Each situation presents its own set of conditions which must be accommodated by the methods and apparatus used to separate the gas and blood.
In a first case of use, blood is degasified while it is drawn in relatively small and strongly varying amounts from a patient, in particular through a suction tube from the site of a wound which may have been caused by an accident or an operation. Vacuum for drawing the blood is produced by a suction pump, generally a roller pump. The suction pump not only draws off blood but frequently also draws off air at the place where blood is being removed from the patient. The amount of air which is drawn off is relatively large, as compared with the amount of blood being drawn off, for instance five parts of air to one part of blood (parts by volume). The blood-removal rate is relatively low, for example 100 to 600 milliliters per minute, and it varies greatly. The suction flow speed of the blood is also relatively low and varies greatly.
The air bubbles contained in the blood are frequently relatively large. Their size ranges from the micrometer range to the millimeter range. The present invention concerns this first case of use, namely degasifying blood while it is being drawn off from a patient.
The second case of use concerns administering blood to a patient, for instance upon dialysis or after a large loss of blood in an accident. Blood which is administered to a patient cannot be fed by suction but can only be administered to the patient by the pressure of a pressure pump. The blood rate, for instance three liters per minute, and the speed of conveyance of the blood are relatively high and substantially constant. The amount of air in the blood is, however, slight, for instance only 50.times.10.sup.-6 parts by volume of air to one part by volume of blood. The air has the form of only very small bubbles in the micrometer range.
The third case of use concerns the handling of blood outside a patient and independently of the patient. Here, similar to the second case, there are substantially constant blood flow quantities and constant speeds of flow of blood.
For the first case of use, U.S. Pat. No. 3,785,380 discloses a blood removal device which consists of a cylindrical housing in which microporous filter material is present for filtering air bubbles and other impurities out of a stream of blood drawn off from a patient, a blood suction tube at a front end of the housing, and a blood suction line at the rear end of the housing.
Literature concerning the second case of use includes GB-A-2 063 108 which shows a blood degasification device for removing bubbles of gas which can be so small as to lie in the micro range, for instance having a diameter of only 40 microns. This blood suction device has a vertically arranged cylindrical cyclone chamber, with a tangential inlet at the upper end of the chamber, and a blood outlet at the lower end of the chamber arranged tangentially opposite the direction of rotation of the cyclone. An air vent tube extends in downward direction in the axis of rotation of the cyclone to a level below the blood inlet into the cyclone chamber. A second venting means is in the form of a radial hole in an upper extension of the cyclone chamber above the blood inlet. A second tube extends through the entire cyclone chamber along the axis of rotation of the cyclone and through a part of the first mentioned tube. The second tube serves so that air bubbles collect on its outer surface and can rise upward. German DE-A-43 29 385 describes an air separator which is an improvement on the one described in the above described GB-A-2 063 108. In the German publication, the blood inlet and blood outlet are arranged axially to each other at the ends of a cylindrical eddy chamber which face each other. The blood inlet is formed by a guide blade body and there is a filter candle in front of the blood outlet. Ascending bubbles of blood enter into a section of the eddy chamber which is located above the guide blade body and in which an air cushion is formed which is vented by a hole. German DE-C-36 41 644 shows a blood flow chamber having a blood inlet at the mid-height of the chamber and a blood outlet channel immersed in the flow chamber. Air bubbles contained in the blood can rise upward only due to the Archimedean buoyancy force.
Furthermore, German DE-C-36 24 363 and U.S. Pat. No. 5,451,321 show devices with microporous filter material for filtering gas bubbles or other blood impurities out of a stream of blood.